Polyketides are a large, structurally diverse family of natural products. Polyketides possess a broad range of biological activities including antibiotic and pharmacological properties. For example, polyketides are represented by such antibiotics as tetracyclines and erythromycin, anticancer agents including daunomycin, immunosuppressants, for example FK506 and rapamycin, and veterinary products such as monensin and avermectin. Polyketides occur in most groups of organisms and are especially abundant in a class of mycelial bacteria, the actinomycetes, which produce various polyketides.
Polyketide synthases (PKSs) are multifunctional enzymes related to fatty acid synthases (FASs). PKSs catalyze the biosynthesis of polyketides through repeated (decarboxylative) Claisen condensations between acylthioesters, usually acetyl, propionyl, malonyl or methylmalonyl. Following each condensation, they introduce structural variability into the product by catalyzing all, part, or none of a reductive cycle comprising a ketoreduction, dehydration, and enoylreduction on the .beta.-keto group of the growing polyketide chain. PKSs incorporate enormous structural diversity into their products, in addition to varying the condensation cycle, by controlling the overall chain length, choice of primer and extender units and, particularly in the case of aromatic polyketides, regiospecific cyclizations of the nascent polyketide chain. After the carbon chain has grown to a length characteristic of each specific product, it is released from the synthase by thiolysis or acyltransfer. Thus, PKSs consist of families of enzymes which work together to produce a given polyketide. It is the controlled variation in chain length, choice of chain-building units, and the reductive cycle, genetically programmed into each PKS, that contributes to the variation seen among naturally occurring polyketides.
Two general classes of PKSs exist. One class, known as Type I PKSs, is represented by the PKSs for macrolides such as erythromycin. These "complex" or "modular" PKSs include assemblies of several large multifunctional proteins carrying, between them, a set of separate active sites for each step of carbon chain assembly and modification (Cortes, J. et al. Nature (1990) 348:176; Donadio, S. et al. Science (1991) 252:675; MacNeil, D. J. et al. Gene (1992) 115:119). Structural diversity occurs in this class from variations in the number and type of active sites in the PKSs. This class of PKSs displays a one-to-one correlation between the number and clustering of active sites in the primary sequence of the PKS and the structure of the polyketide backbone.
The second class of PKSs, called Type II PKSs, is represented by the synthases for aromatic compounds. Type II PKSs have a single set of iteratively used active sites (Bibb, M. J. et al. EMBO J. (1989) 8:2727; Sherman, D. H. et al. EMBO J. (1989) 8:2717; Fernandez-Moreno, M. A. et al. J. Biol. Chem. (1992) 267:19278).
In contrast, fungal PKSs, such as the 6-methylsalicylic acid PKS, consist of a single multi-domain polypeptide which includes all the active sites required for the biosynthesis of 6-methylsalicylic acid (Beck, J. et al. Eur. J. Biochem. (1990) 192:487-498; Davis, R. et al. Abstr. of the Genetics of Industrial Microorganism Meeting, Montreal, abstr. P288 (1994)).
Streptomyces is an actinomycete which is an abundant producer of aromatic polyketides. In each Streptomyces aromatic PKS so far studied, carbon chain assembly requires the products of three open reading frames (ORFs). ORF1 encodes a ketosynthase (KS) and an acyltransferase (AT) active site; ORF2 encodes a PKS chain length determining factor (CLF); and ORF3 encodes a discrete acyl carrier protein (ACP).
Streptomyces coelicolor produces the blue-pigmented polyketide, actinorhodin. The actinorhodin gene cluster (act), has been cloned (Malpartida, F. and Hopwood, D. A. Nature (1984) 309:462; Malpartida, F. and Hopwood, D. A. Mol. Gen. Genet. (1986) 205:66) and completely sequenced (Fernandez-Moreno, M. A. et al. J. Biol. Chem. (1992) 267:19278; Hallam, S. E. et al. Gene (1988) 74:305; Fernandez-Moreno, M. A. et al. Cell (1991) 66:769; Caballero, J. et al. Mol. Gen. Genet. (1991) 230:401). The cluster encodes the PKS enzymes described above, a cyclase and a series of tailoring enzymes involved in subsequent modification reactions leading to actinorhodin, as well as proteins involved in export of the antibiotic and at least one protein that specifically activates transcription of the gene cluster. Other genes required for global regulation of antibiotic biosynthesis, as well as for the supply of starter (acetyl CoA) and extender (malonyl CoA) units for polyketide biosynthesis, are located elsewhere in the genome.
The act gene cluster from S. coelicolor has been used to produce actinorhodin in S. parvulus. Malpartida, F. and Hopwood, D. A. Nature (1984) 309:462. Bartel et al. J. Bacteriol. (1990) 172:4816-4826, recombinantly produced aloesaponarin II using S. galilaeus transformed with an S. coelicolor act gene cluster consisting of four genetic loci, actI, actIII, actIV and actVII. Hybrid PKSs, including the basic act gene set but with ACP genes derived from granaticin, oxytetracycline, tetracenomycin and frenolicin PKSs, have also been designed which are able to express functional synthases. Khosla, C. et al. J. Bacteriol. (1993) 175:2197-2204. Hopwood, D. A. et al. Nature (1985) 314:642-644, describes the production of hybrid polyketides, using recombinant techniques. Sherman, D. H. et al. J. Bacteriol. (1992) 174:6184-6190, reports the transformation of various S. coelicolor mutants, lacking different components of the act PKS gene cluster, with the corresponding granaticin (gra) genes from S. violaceoruber, in trans.
However, no one to date has described the recombinant production of polyketides using genetically engineered host cells which substantially lack their entire native PKS gene clusters.